1. Field of the Invention
This invention relates generally to the field of disinfectants, and relates more specifically to a method and apparatus for producing hypochlorous acid solutions and maintaining hypochlorous acid concentrations by manipulating the pH of the solution. In particular the invention relates to the use of acids to produce or maintain hypochlorous acid levels in aqueous solutions.
2. Background of the Invention
Chlorination is known method for killing undesirable microorganisms. Chlorine may be provided in multiple forms including chlorine gas (Cl2), sodium hypochlorite liquid, calcium hypochlorite powder or granules, or isocyurantes. Chlorine gas (Cl2) is a relatively cheap and highly effective antimicrobial agent; however, it is also a highly toxic and corrosive gas. Hypochlorites such as NaOCl or Ca(OCl)2 are a much safer alternative, but are considerably more expensive that gaseous chlorine. Finally, hypochlorite solutions (i.e., bleach) may also be utilized, however these are rarely used in large scale water treatment applications because they are bulky and expensive. Regardless of the chlorine source, hypochlorous acid (HOCl) and the hypochlorite ion (OCl−) are the final desirable antimicrobial products.
One method of forming HOCl occurs when Cl2 is dissolved in water. The reaction proceeds according to the following equation:Cl2+H2OHOCl+H++Cl−  (1)
Another method for producing HOCl uses metal hypochlorites dissolved in water. The reaction proceeds according to the following equation:
 NaOCl+H2ONaOH+HOCl  (2)
This method is generally utilized by common household hypochlorites and generates HOCl on a relatively small scale.
HOCl is a weak acid and will dissociate. In aqueous solution, HOCl and OCl− are generally present in a pH dependent equilibrium:HOClH++OCl−pKa=7.53  (3)
At low pH, HOCl is the predominant form, while at high pH, OCl− predominates. The HOCl form is about 80 times more effective than OCl− for killing microorganisms because HOCl crosses cell membranes easier than the hypochlorite ion. Accordingly, it would be desirable to control the pH of the chlorinated solution to increase the antimicrobial effectiveness of the chlorination process.
Processes and systems for adjusting the pH of a water stream are known in the art. U.S. Pat. No. 5,487,835 to Shane discloses a method and apparatus for controlling the pH of a water stream using carbon dioxide. Several methods currently are used to inject carbon dioxide into water. The most accepted method is to inject the carbon dioxide into the water by a direct gas feed through some type of diffusion system in a recarbonation basin; in effect, a bubbler. A mechanical mixer can be used in combination with this method for better efficiency. Another method for injecting carbon dioxide into water is to aspirate the carbon dioxide into a stream of water using a venturi type eductor. In this method, the carbon dioxide is injected into the stream of water and carried along with the stream of water to a grid system located in a basin or a pipeline.
Both the direct gas feed method and the venturi method of injecting carbon dioxide gas into water allow for the control of the pH and the stabilization of the treated water. However, it is difficult to control the efficiency of the carbon dioxide gas usage. Both of these processes require the use of a relatively large contact basin, a relatively long contact time or large amount of carrier water, all of which inherently are inefficient.
The formation of hypochlorous acid is also known in the art. For example, U.S. Pat. No. 4,017,592 to Penard et al. discloses a process for producing aqueous hypochlorous acid solution using a gas, liquid, solid phase system. U.S. Pat. No. 4,146,578 to Brennan et al. discloses a process for preparing hypochlorous acid wherein gaseous chloride is reacted with an aqueous solution of an alkali metal hydroxide in a finely divided mist at an elevated temperature to prevent condensation. U.S. Pat. No. 4,147,761 to Wojtowicz et al. discloses a hypochlorous acid process using a sweep reactor in which gaseous chloride is passed rapidly across the surface of an agitated aqueous solution of alkali metal hydroxide at a reduced temperature.
U.S. Pat. No. 4,190,638 to Hoekje et al. discloses the production of hypochlorous acid wherein a precipitate formed on a carbonating electrolytic cathode cell liquor is contacted in a fluidized bed with a mixture of gaseous chlorine and water vapor. The exit gas from the fluid vapor is absorbed in water. U.S. Pat. No. 4,504,456 to Yant et al. discloses a process and apparatus for forming hypochlorous acid by hydrolyzing chlorine with steam and water vapor and solid-gas stripping of the co-generated hydrochloric acid vapor. Hypochlorous acid vapor is dissolved into an aqueous product solution.
U.S. Pat. No. 5,037,627 to Melton et al. discloses a hypochlorous acid process by reacting an aqueous solution of an alkali metal hydroxide in droplet form with gaseous chlorine to produce hypochlorous acid vapor and solid alkali metal chloride particles. U.S. Pat. No. 5,322,677 to Shaffer et al. discloses a process for producing a concentrated aqueous hypochlorous acid solution by reacting droplets of an alkali metal hydroxide solution containing greater than 50 percent by weight of the alkali metal hydroxide with chlorine gas.
Despite the several known processes for producing hypochlorous acid, there remains a need for a quick, safe, and efficient process for producing hypochlorous acid solutions suitable for use as a disinfectant.